Drain rutabagas; return to same pot. Mash to coarse puree. Stir over medium heat until excess moisture evaporates, 5 minutes. Add cream, butter, and thyme. Mix in pears and any juices from baking sheet. Season with salt and pepper. DO AHEAD: Can be made 1 day ahead. Transfer to microwave-safe bowl. Cover; chill. Rewarm at 1-minute intervals.